Superplastic forming of metals is well known in the art whereby complex shapes are formed from metal at elevated temperature utilizing the superplastic forming characteristics of the metal to avoid tearing and other problems in forming complex shapes. Superplastic forming can be viewed as an accelerated form of high-temperature creep and occurs much like sagging or creep forming. In the case of aluminum alloys, superplastic forming is normally performed at temperatures above 700.degree. F., typically in the range of about 900.degree. to 1000.degree. F. or a little higher. At this temperature, the metal creeps and can be moved by shaping operations at relatively low stress levels, the stress at which the metal starts to move easily or flow being referred to as the "flow stress". Superplastic forming is recognized as being able to produce intricate forms or shapes from sheet metal and offers the promise of cost savings. For instance, an airplane member previously made by stamping several parts from sheet and then joining the separate parts together into a more complex shape can be formed from a single piece of metal by superplastic forming techniques. Alternatively, the part may be superplastically formed by the forging process whereby the starting stock may be either an ingot or a semi-fabricated, hot worked product. However, the superplastic forming techniques themselves are time-consuming in that like any form of creep forming, the metal flowing operation proceeds relatively slowly in comparison with high-speed press forming. Substantial cost-savings and benefits could be realized if the aluminum alloy to be superplastically formed could be made to flow faster at a given temperature or be superplastically formed at a lower temperature or both without tearing or rupturing.
There are a number of approaches taken to enhance superplastic forming. Some of these approaches are directed to manipulations in the superplastic forming operation to enhance that operation or alleviate problems therein largely by controlling the flow of the metal during forming. Examples of such are shown in U.S. Pat. Nos. 3,997,369, 4,045,986, 4,181,000, and 4,516,419, all incorporated herein by reference. Another approach is directed to the metal to be superplastically formed. It has long been recognized that fine grain size enhances forming operations including superplastic forming operations. Some examples of efforts to achieve fine grain size are shown in U.S. Pat. Nos. 3,847,681 and 4,092,181. One approach to achieving fine grain size which was old as far back as the 1960's includes imparting substantial working effects such as cold work to aluminous metal followed by rapid heating to recrystallization temperature. However, despite the various approaches taken to improve either the superplastic forming operation or the metal stock going into the operation, there remains substantial room for improvement and an alloy which would enable the superplastic forming operation to proceed faster or at a lower temperature is both desirable and sought after.